Mobile communication systems require a random access mechanism by which mobile terminals can access a network. In one example of a random access mechanism which is under consideration for the UMTS LTE (Universal Mobile Telecommunication System Long Term Evolution) a mobile terminal transmits a signature on an asynchronous random access channel (RACH). The mobile terminal selects a particular signature from a larger set (e.g. of 64), which enables a few bits (e.g. 6) of information to be conveyed. One requirement is that the collision probability (i.e. probability that two mobile terminals use the same signature at the same time) should be sufficiently low. A further requirement is that the resulting waveforms have low cross-correlation.
The bit pattern defining the signature can be considered in terms of one or more data fields. Some examples of the potential contents of the data fields are:                Pseudo-random data (in the form of a temporary mobile terminal identity to reduce the risk of collision)        Channel quality indicator (CQI) or pathloss measurement        Reason for RACH access.The CQI and Reason fields may also help to reduce collisions if they are sufficiently uncorrected between terminals.        
Following the detection of the signature by the network, or more specifically the access point, further signalling takes place in both uplink (mobile terminal to network) and downlink (network to mobile terminal), for example to allocate resources for data transmission.
The access point needs channel state information in order to choose transmission characteristics (e.g. rate, power, beamforming, pre-coding) in order to maximize the efficiency of the downlink transmissions in the final stages of the random access process, and later. The example mechanism described provides only a small number of useful data bits which can be sent in the first transmission, and so the channel cannot be described very accurately. It is also desirable to minimise the potential collision probability.